So why wasn’t she just with the cop from the beginning?

she had commitment issues.

To show a richer web of relationships. The film shows a breakdown of virtually ever single relationship she has (other than the ex).

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She gave a speech near the end apologizing to him for bottling up her emotions, putting up a wall and being selfish.

Seems like she left him because of her own shortcomings. She'd been getting therapy in the past for her trauma due to her mothers suicide. She was a damaged person like many of the other victims. The entity seems to feed of their trauma, it states that her mind is very inviting. For example the professor who killed himself with a hammer had never gotten over the loss of his brother (car accident if I recall correctly?) as stated by his wife. These traumatized people seem particularly susceptible to being chosen by the entity.

This film reminds me a lot of Ringu. There, the protagonist, a woman is also caught in a chain curse and seeks the help of her estranged husband. They clearly care about each other and like this, she ensnares her ex into the curse as well.

Went black, couldn't go back.